Remote Push install

I've just upgraded the console to version 4.0.138.0 and am now unable to add new clients for push install. I've searched all the groups for my new PCs but some do appear immediately so I used to just add their ip address and it would work fine. Now I can't, and some clients take an hour to appear!

I forced an AD sync in Group Manager and some still don't appear, even though my AD MMC shows them fine.